There are various types of writing instruments such as fountain pens, mechanical pencils and others. Ball-point pens are one of these. Of the ball-point pens, pressurized ball-point pens which are filled up with pressurized gas are known, which have a structure, as disclosed in U.S. Pat. No. 4,472,462 or U.S. Pat. No. 3,997,273, including a joint as a holder for joining the rear end of a writing point assembly as a writing part and the open front part of an ink tube and an inner barrel fitted to the joint by bonding, tight fitting or other methods, wherein the inner barrel is filled up with a pressurized gas such as air, nitrogen gas or the like.
In such pressurized ball-point pens, mere press fitting of the point assembly into the joint or ink tube poses the risk of dislodging the writing assembly due to sealed gas pressure, ambient temperature influences or creep with passage of time. There is also a gas leakage problem occurring due to micro damages during assembly or other reasons. Similarly, even in the case of ordinary writing instruments other than the pressurized ball-point pens, there are cases where the resin joint or ink tube may crack when the point assembly with a writing part is press fitted so that ink will not leak out.
In order to solve these problems, conventionally a flexible resin that is inferior in the other characteristics, has been used, or a method of forming an annular groove in the writing point assembly at the part to be inserted into the ink tube has been adopted. Alternatively, a special shape may be formed so that molten resin by the effect of ultrasonic waves is flowed into annular grooves etc. Also, the holder may be welded by ultrasonic heating to the front end of the barrel to which the writing point assembly is fitted, so that the writing point assembly is fixed by a joint that that externally fits on the writing point assembly.
However, when ultrasonic waves are used for welding, the writing point assembly is affected by ultrasonic waves, possibly causing damage to the writing part and inducing problems such as dislodging of the ball from the tip of the writing point assembly, ink blotting or ink flushing, or splitting during writing. Further, it is impossible to maintain the initial strength of the writing point assembly at the part that is inserted into the ink tube, due to external force, influence from ambient temperature, creep deformation and cracks and others of the material itself. As a result, there occur problems such as dislodging of the writing point assembly, ink leakage, self-heating of the entire writing instrument, instability of welding and the like.
Further, in the conventional writing instruments, since the inner barrel is merely fitted to the joint, there is a risk of the pressurized gas leaking out if the joint is not fixed fast to the inner barrel. This also becomes a cause that may induce ink leakage and other problems.
In order to solve these problems, forcible press fitting or sealant application may be done for a case of metal. For a case of resin, the joint and the inner barrel may be joined by firm fitting using ultrasonic waves or the like. These methods however need difficult assembly or high cost, and also entails the risk of dislodging of the ball held by the writing point assembly, ink blotting and flushing and other new critical problems occurring.